"But DH says something similar to Aussie - why bother, bascially, as she won't change. And that's where I sigh because I truly don't know sometimes - maybe just hope that she will understand she needs to stop pulling this stuff."
I understand how upset both you and your DH are at going to all this effort and then having it rejected. It sounds like your MIL is one of those people who creates difficulties, and is surrounded by relatives who let her get away with her ridiculous behavior. I have one of those in my own family. There is an alternative to the cut direct and the other option of letting her get away with things. Cut way back on contact with her, refuse to get involved with planning anything with her, when she messes up events give her an honest, low key comment about why you are upset. If she doesn't like it, tough.
When and if you have kids, it will be more important than ever to stop putting up with her guff. Again, solution, limit contact if she can't behave.